This invention relates to the polymerization of elastomeric isoolefinic homopolymers and copolymers, especially the polymerization reaction required to produce the isobutylene-isoprene form of butyl rubber. More particularly, the invention relates to a method of stabilizing against agglomeration the polymerization slurries used in the preparation of such polymers, the medium, or diluent of such slurries being methyl chloride or certain other polar chlorinated hydrocarbon diluents.
The term "butyl rubber" as used in the specification and claims means copolymers of C.sub.4 -C.sub.7 isoolefins and C.sub.4 -C.sub.14 conjugated dienes which comprise about 0.5 to about 15 mole percent conjugated diene and about 85 to 99.5 mole percent isoolefin. Illustrative examples of the isoolefins which may be used in the preparation of butyl rubber are isobutylene, 2-methyl-1-propene, 3-methyl-1-butene, 4-methyl-1-pentene and .beta.-pinene. Illustrative examples of conjugated dienes which may be used in the preparation of butyl rubber are isoprene, butadiene, 2,3-dimethyl butadiene, piperylene, 2,5-dimethylhexa-2,4-diene, cyclopentadiene, cyclohexadiene and methylcyclopentadiene. The preparation of butyl rubber is described in U.S. Pat. No. 2,356,128 and is further described in an article by R. M. Thomas et al. in Industrial and Engineering Chemistry, vol. 32, pp. 1283 et seq., October, 1940. Butyl rubber generally has a viscosity average molecular weight between about 100,000 to about 800,000, preferably about 250,000 to about 600,000 and a Wijs Iodine No. of about 0.5 to 50, preferably 1 to 20.
The term isoolefin homopolymers as used herein is meant to encompass those homopolymers of C.sub.4 -C.sub.7 isoolefins particularly polyisobutylene, which have a small degree of terminal unsaturation and certain elastomeric properties. The principal commercial forms of these butyl rubber and isoolefin polymers such as isobutylene-isoprene butyl rubber and polyisobutylene, are prepared in a low temperature cationic polymerization process using Lewis acid type catalysts, typically aluminum chloride being employed. Boron trifluoride is also considered useful in these processes. The process extensively used in industry employs methyl chloride as the diluent for the reaction mixture at very low temperatures, that is less than minus 90.degree. C. Methyl chloride is employed for a variety of reasons, including the fact that it is a solvent for the monomers and aluminum chloride catalyst and a nonsolvent for the polymer product. Also, methyl chloride has suitable freezing and boiling points to permit, respectively, low temperature polymerization and effective separation from the polymer and unreacted monomers.
The slurry polymerization process in methyl chloride offers a number of additional advantages in that a polymer concentration of approximately 30% by weight in the reaction mixture can be achieved, as opposed to the concentration of only about 8% to 12% in solution polymerization. Also, an acceptable relatively low viscosity of the polymerization mass is obtained enabling the heat of polymerization to be removed more effectively by heat exchange. Slurry polymerization processes in methyl chloride are used in the production of high molecular weight polyisobutylene and isobutylene-isoprene butyl rubber polymers.
Notwithstanding the widespread use of the slurry polymerization process in methyl chloride, there are a number of problems in carrying out this process which are related to the tendency of the polymer product particles to agglomerate, and thereby destabilize the slurry dispersion. The rate of agglomeration increases rapidly as reaction temperature approaches -90.degree. C. It is not possible to maintain a stable slurry above -80.degree. C. These agglomerated particles tend to adhere to and to grow and plate out on all surfaces they contact, such as reactor discharge lines, as well as reactor inlet lines and any heat transfer equipment being used to remove the exothermic heat of polymerization, which is critical since low temperature reaction conditions must be maintained.
Heretofore, no effective technique of stabilizing the slurry has been found other than by operation below -80.degree. C. and with high agitation in the reactor. It has become standard practice to design manufacturing facilities with additional reactor equipment so that the reaction process can be cycled between alternate reactor systems so that at any given time one or more reactors are in the process of being cleaned. If a stable slurry could be produced and maintained in a non-fouling condition, substantial economies in equipment installation and process techniques could be achieved. A further limitation imposed by the tendency of the polymer product particles to agglomerate is the inefficiency of heat exchange, which effectively prevents any attempt to heat exchange the cold reactor effluent with the incoming feed in order to realize savings in the refrigeration energy required.
A general reference text which discusses the theory and principles concerning dispersion polymerization and in particular the use of block and graft copolymers as dispersion stabilizers is "Dispersion Polymerization in Organic Media", edited by K. E. J. Barrett, John Wiley & Sons, 1975. While this text, particularly in Chapter 3, discloses the use of block or graft copolymers having an insoluble component, or anchor group, and a diluent-soluble component in a number of dispersion polymerization processes, no disclosure is made of any stabilizer system useful in the methyl chloride slurry polymerization process for isoolefin homopolymers or butyl rubber copolymers as disclosed in accordance with the present invention.
In published Netherlands Application No. 770760 (1977), filed in the U.S. on June 14, 1976, as Ser. No. 699,300, now U.S. Pat. No. 4,098,980, issued July 4, 1978, Markle et al disclose a non-aqueous dispersion polymerization process for conjugated diolefins in the presence of a block copolymer dispersion stabilizer, at least one block being soluble in the liquid organic dispersion medium and at least another block being insoluble in the dispersion medium. The Markle et al disclosure deals with the polymerization of a conjugated diolefin monomer in a liquid hydrocarbon dispersion medium such as n-butane, neopentane or mixed isomeric pentanes in the presence of a Ziegler-Natta Catalyst. The conjugated diolefins, particularly preferred by Markle et al, are butadiene-1,3, isoprene and piperylene. Markle et al also disclose mixtures of conjugated diolefins.
The process of the present invention is considered distinguished from the disclosure of Markle et al in that it relates to a cationic polymerization carried out in a polar chlorinated hydrocarbon diluent, such as methyl chloride, utilizing stabilizers which are especially effective in that polymerization process. Markle et al deal with anionic polymerization processes conducted in a non-polar liquid hydrocarbon diluent.
So far as the inventors hereof are aware, no effective method for stabilizing methyl chloride slurries, nor slurries in any type of diluent, used in the production of isoolefin polymer products with chemical additive stabilizers is known or disclosed in the prior art.
In accordance with the present invention, there has been discovered a method of stabilizing a polymerization slurry against agglomeration, the slurry containing an isoolefin homopolymer or a butyl rubber copolymer in a polymerization diluent, the diluent being methyl chloride, methylene chloride, vinyl chloride or ethyl chloride, which comprises incorporating into the reaction mixture which comprises the mixture of monomers, catalyst and diluent, or into the polymerization product slurry about 0.05% to 20% by weight, based upon the weight of product isoolefin homopolymer or product butyl rubber copolymer, of a stabilizing agent, the stabilizing agent being (i) a preformed copolymer having a lyophilic, diluent soluble portion and a lyophobic, diluent insoluble, isoolefin homopolymer or butyl rubber soluble or adsorbable portion, the stabilizing agent being capable of forming an adsorbed solubilized polymer coating around the precipitated isoolefin homopolymer or butyl rubber copolymer to stabilize the slurry, or (ii) an in situ formed stabilizing agent copolymer formed from a stabilizer precursor, the stabilizer precursor being a lyophilic polymer containing a functional group capable of copolymerizing or forming a chemical bond with the isoolefin polymer or butyl rubber copolymer being formed in the main polymerization process, the functional group being a cationically active halogen, either pendant or enchained or cationically active unsaturation, the lyophobic portion of the stabilizing agent being the isoolefin homopolymer or butyl rubber copolymer which is being formed in the main polymerization process, the stabilizing agent so formed being capable of forming an adsorbed solubilized polymer coating around the precipitated product polymer to stabilize the product polymer slurry.
The quantity of stabilizing agent set forth above is expressed as a percent by weight of the product isoolefin homopolymer or butyl rubber copolymer. The exact quantity of stabilizer agent added to the reaction mixture is a function of the exact concentration of the feed blend and the estimated degree of conversion of monomers. In a typical butyl rubber reaction process for manufacturing isobutylene-isoprene butyl rubber, the reactor feed blend which is prepared contains about 25% to 35% by weight monomers, and typically 80% to 90% by weight of monomers are converted to polymer product.
The present invention deals with two forms of suitable stabilizing agents, both of which are effective in the polymerization diluent and serve to stabilize the polymerization slurry comprised of the polymer or copolymer particles which are produced in the basic polymerization reaction. As used herein, the term "polymerization diluent" is meant to refer to methyl chloride, methylene chloride, vinyl chloride and ethyl chloride. Methyl chloride is the preferred diluent in all embodiments of this invention.
Utilization of a preformed block or graft copolymer, which is both lyophilic and lyophobic in the presence of the polymerization diluent, involves first providing a suitable copolymer. Generally, a preformed copolymer stabilizer must have a diluent insoluble anchor portion, which is adsorbable or soluble in polymerized isoolefin or butyl rubber, as well as a diluent soluble portion which functions to keep the adsorbed polymer dispersed in the polymerization diluent.
The preformed block or graft copolymer stabilizer, subject to certain limitations as set forth below, may be added to the reaction mixture and can be present throughout the polymerization reaction to prevent agglomeration at reaction temperatures. Alternatively, a portion of the preformed stabilizer can be added to the reaction mixture and additional stabilizer can be injected into the reactor effluent lines to prevent agglomeration in downstream equipment.
Certain categories of preformed stabilizers, while being effective as slurry stabilizers in the present invention, are preferably added upon completion of the polymerization reaction. Thus, they are preferably added to the reactor effluent in order to prevent agglomeration during the final stages of processing. These preformed stabilizer copolymers are defined as those containing a substantial amount of cationically active unsaturation or functional groups, the functional groups being hydroxyl, ester, ketone, amino, aldehyde, nitrile, amido, carboxyl, sulfonate, mercaptan, ether, anhydride, nitro, active allylic or active tertiary halogen. Functional groups with a low degree of cationic activity, or present at low concentrations may be only marginally active under typical butyl polymerization conditions. The degree to which these groups participate in the polymerization reaction to form chemical bond attachments to the product polymer depends upon the polymerization conditions (monomer conversion, temperature, steady-state monomer and stabilizer concentrations, etc.). Under some polymerization conditions marginally active functional groups are effectively inert so that no appreciable chemical bond attachment to the product polymer occurs and the preformed stabilizing agent primarily functions by adsorption on the product slurry particles. Under other polymerization conditions appreciable chemical bond attachment to the product polymer can occur through these marginally cationically active groups. To the extent that chemical attachment occurs, the preformed copolymer stabilizer is acting as a functional lyophile and forms, in situ, a new stabilizer with the product polymer as the lyophobe.
If cationic activity is even higher, then extensive and multiple attachments to the product polymer can occur and the stabilizer may become undesirable for use during polymerization. Given that a stabilizing agent as disclosed herein functions effectively to stabilize the product polymer slurry, its suitability as a preformed stabilizing agent will be predicated, in part, on formation of a gel-free polymer product; the absence of gelled material on reactor surfaces is also a desirable feature. The end use application to which the product polymer will be put can have significant impact on the choice of stabilizing agent and its tendency to result in gel formation under the polymerization conditions. Some applications may require that the product be gel free, whereas in others the presence of gel may be tolerable or even preferable (e.g., mastics).
The lyophilic portion of the preformed copolymer stabilizing agent employed in the present invention must be completely soluble in, or miscible with, the polymerization diluent. A suitable criterion is that the lyophilic portion have a Flory-Huggins interaction parameter with the polymerization diluent of less than 0.5 or a Flory solvency coefficient with the polymerization diluent greater than 1.
Suitable lyophilic polymers which meet these requirements and which do not adversely affect the catalyst or polymerization conditions include polystyrene, polyvinyl chloride, polyvinyl bromide and neoprene, with the preferred lyophilic portion being polystyrene, polyvinyl chloride, or polyvinyl bromide. Also suitable are substituted styrene lyophiles such as mono-, di- and tri-substituted styrenes, the substituents being halogen, such as chlorine, or lower (C.sub.1 -C.sub.5) alkyl groups, as illustrated by alpha-methyl styrene, para-t-butyl-styrene, p-chlorostyrene and similar ring chlorinated styrenes. It is also suitable to employ as the lyophilic portion combinations of two suitable lyophilic polymers such as copolymers of styrene and vinyl chloride. Thus, the term "lyophilic portion" as used herein is meant to encompass a portion composed of one or more monomers which meet the criteria for suitable lyophiles in the practice of the present invention. This lyophilic portion should have a degree of polymerization (D.P.) of at least about 20 and up to about 5,000 or 6,000.
A number of significant factors influence the selection of the lyophobic portion of the stabilizing agent. The lyophobic portion is insoluble in polymerization diluent but should have a high affinity for the product polymer so that it is adsorbed or otherwise bonded onto the polymer particle. A lyophobic portion composed of the same material being produced in the cationic Lewis Acid catalyzed polymerization reaction, such as isobutylene homopolymer or isobutylene-isoprene butyl copolymer, makes an ideal lyophobic portion in the preformed stabilizer agent employed in the present invention. Suitable lyophobic materials generally include diluent insoluble polymers having a solubility parameter less than about 8 and a degree of polymerization (D.P.) of at least about 10. Suitable materials include polyisolefins generally of C.sub.4 -C.sub.7 isoolefins, such as polyisobutylene, butyl rubber copolymers generally, such as isobutylene-isoprene butyl rubber, polybutadiene, polyisoprene, ethylene/propylene copolymers, EPDM terpolymers, hydrogenated diene polymers, e.g. hydrogenated polybutadiene, SBR Rubbers, which are styrene/butadiene random copolymers of low styrene content and polydimethyl silicone. A particularly preferred preformed stabilizer for use in the production of isobutylene-isoprene butyl rubber is a preformed block copolymer stabilizer agent composed of an isobutylene-isoprene portion block or graft with about 20 to 80 weight percent styrene block or graft. Also preferred is a styrene-EPDM preformed stabilizer.
In situ formation of the stabilizer utilizes a lyophilic polymer component having a functional group capable of reacting with the isoolefin or butyl rubber polymer being formed in the main polymerization process. In this embodiment, the polymer being prepared becomes the lyophobic portion of the copolymer stabilizer.
The in situ method of preparing the stabilizer copolymer in the present invention involves first providing a stabilizer precursor which is a lyophilic polymer having a functional group capable of copolymerizing or otherwise reacting with the isoolefin polymer, e.g. polyisobutylene or isobutylene-isoprene, being formed in the main polymerization reaction to form the block or graft copolymer stabilizer in accordance with the present invention. The functional groups may be cationically active pendant or enchained halogen, preferably chlorine, or cationically active unsaturation.
Formation of these stabilizer precursors may be accomplished through free radical polymerization of a lyophile such as styrene in the presence of carbon tetrachloride or by free radical copolymerization of a lyophile such as styrene with vinyl benzyl chloride. These stabilizer precursors will contain active halogen which lead to in situ formation of the stabilizer copolymer in the present invention through a chain transfer or co-initiation reaction mechanism.
Formation of a stabilizer precursor containing cationically active unsaturation as the functional group in the lyophile can be accomplished by anionically polymerizing a lyophile such as styrene and capping it with vinyl benzyl chloride or methallyl chloride whereby the residue of this vinyl benzyl chloride or methallyl chloride yields cationically active unsaturation. This stabilizer precursor then forms the stabilizer copolymer of the present invention by copolymerizing with the isoolefin polymer or butyl rubber copolymer being formed in the main polymerization reaction.
The above embodiments may be illustrated by first considering lyophilic polystyrenes having a reactive chlorine as an end group: ##STR1## or an active enchained chlorine pendant to a styrene polymer chain ##STR2##
Lyophilic polystyrene stabilizer precursors represented above containing terminal or enchained active chlorine can be prepared, respectively by polymerizing styrene using free radical catalysts in the presence of carbon tetrachloride which acts as a transfer agent to yield a chlorine capped polystyrene and by copolymerizing styrene with a minor amount of vinyl benzyl chloride to form a polystyrene containing enchained vinyl benzyl chloride.
These lyophilic portions containing an active halogen will incorporate polystyrene into a polyisolefin or butyl rubber copolymer chain by a transfer mechanism or co-initiation mechanism. Chain transfer is best illustrated by reference to an isobutylene polymerization. In this reaction a growing isobutylene carbonium ion abstracts the active halogen as a Cl.sup..crclbar. from the lyophilic polystyrene to yield a Cl.sup..crclbar. capped polyisobutylene chain and a polystyryl carbonium ion which, in the presence of isobutylene monomer, propagates to form a stabilizer block copolymer consisting of a polystyrene chain attached to an isobutylene chain. A graft copolymer can also be formed and in the present invention the term stabilizer copolymer or stabilizer polymer may include blocks, grafts, mixtures thereof or other configurations resulting from copolymerization reactions. The same mechanism would apply to utilization in isobutylene-isoprene polymerization. The mechanism is illustrated for reaction with polyisobutylene by the following equations: ##STR3##
Co-initiation may be illustrated with reference to the following equations showing the AlCl.sub.3 polymerization of isobutylene where the stabilizer precursor is a chlorine-containing polystyrene. ##STR4##
Stabilization of the polymerization slurry can be accomplished utilizing as the stabilizer precursor an anionically polymerized lyophile, such as polystyrene, capped with the residue of vinyl benzyl chloride molecule or a methallyl chloride molecule represented respectively by formulas I and II below: ##STR5## .eta. being an integer such that the Mn of the polystyrene chain is about 25,000 to 75,000.
In this embodiment of the present invention, the functional lyophile as illustrated by polystyrene is capable of copolymerizing with isoolefin through the residue of the vinyl benzyl or methallyl unit, which contains cationically active unsaturation. Stabilization is effected by linking the diluent soluble polymer chain to the isoolefin polymer or butyl rubber copolymer as it is formed in the polymerization process. A vinyl benzyl chloride capped polystyrene is especially preferred in the stabilization of methyl chloride slurries containing isobutylene-isoprene butyl rubber copolymer and this stabilizing agent is prepared by anionically polymerizing styrene to a molecular weight of 25,000 to 75,000 in the presence of n-butyl lithium catalyst and then adding vinyl benzyl chloride to cap the living polystyrene chain and precipitate lithium chloride to form the stabilizing agent set forth in formula I above.
Employment of a stabilizer precursor comprising the diluent soluble polymer with a functional group being capable of forming a covalent chemical bond with the isoolefin unit in the polymer product, that is, with isoolefin homopolymer or with the isoolefin portion of butyl rubber copolymer, means that the insoluble or lyophobic portion is not formed until the stabilizer precursor becomes attached to an isoolefin unit during polymerization. Thus the stabilizing molecule is formed in situ during the polymerization process. Selection of the lyophilic portion is governed by the same considerations, including the degree of polymerization values, described above when the preformed block copolymer stabilizing agent is used. Thus, suitable polymerization diluent soluble polymers include polystyrene, polyvinyl chloride, polyvinyl bromide, neoprene and the substituted styrenes as described hereinabove, with polystyrene being particularly preferred.
In using this stabilizing method, it is important that the functional group be active under the cationic polymerization conditions and that the stabilizing agent and functional group not interfere with any aspect of the basic polymerization process. When a preformed copolymer is used, its effectiveness is not dependent upon in situ completion of the formation of the stabilization agent, but, as described above, some degree of in situ participation of the preformed copolymer can occur.
Suitable lyophilic polystyrenes with functional groups capable of bonding with the product polymer and especially with an isobutylene unit in preparation of polyisobutylene homopolymer or isobutylene-isoprene butyl rubber copolymer are those functional polystyrenes having a number average (Mn) molecular weight in the range of about 5,000 to 150,000 and preferably in the range of about 25,000 to 75,000.
The process of the present invention offers a number of significant advantages resulting from the achievement of a stabilized butyl rubber slurry. These include elimination of reactor equipment fouling and plugging, the ability to operate at higher slurry concentrations, increased reactor production rates, the capability of refrigeration recovery by heat exchange of reactor effluent with incoming reactor feed, increased reactor run length time as well as the ability to polymerize at warmer reactor temperatures.
A further embodiment of the present invention comprises the stabilized slurries of isoolefin homopolymer or butyl rubber copolymer prepared in accordance with the present invention containing up to about 50% by weight isoolefin homopolymer or butyl rubber copolymer, particularly a stabilized slurry of isobutylene-isoprene butyl rubber in methyl chloride, said slurry containing up to about 50% by weight butyl rubber, or a slurry containing up to about 50% by weight polyisobutylene.
A further embodiment of the present invention is a novel method of preparing non-agglomerating homopolymers of C.sub.4 -C.sub.7 isoolefins and butyl rubber copolymers by polymerizing the corresponding monomers at temperatures from about -90.degree. C. to about -20.degree. C. in the presence of a Lewis Acid cationic polymerization catalyst in a polymerization diluent selected from the group consisting of methyl chloride, methylene chloride, vinyl chloride and ethyl chloride in the presence of a stabilizer, the stabilizer being either (i) a preformed copolymer having a lyophilic, diluent soluble portion and a lyophobic diluent insoluble but isoolefin or butyl rubber soluble or adsorbable portion or (ii) an in situ formed stabilizer copolymer formed from a stabilizer precursor which is incorporated into the reaction mixture, the stabilizer precursor being a lyophilic polymer containing a functional group capable of copolymerizing or otherwise reacting with the isoolefin or butyl rubber copolymer being formed in the main polymerization process, the functional group being a cationically active pendant or enchained halogen or cationically active unsaturation, the lyophobic portion of the stabilizing agent being the isoolefin or butyl rubber polymer formed in the main polymerization process.
A particular point of novelty is the capability to form non-agglomerating isoolefin homopolymer or butyl rubber copolymer at temperatures of from about -90.degree. C. to -20.degree. C. utilizing AlCl.sub.3 as well as other cationic Lewis Acid polymerization catalysts such as aluminum alkyls, as exemplified by aluminum ethyldichloride, TiCl.sub.4, BF.sub.3, SnCl.sub.4, AlBr.sub.3 and other Friedel-Crafts catalysts.
A particularly preferred embodiment of the present invention resides in the preparation of non-agglomerating isobutylene-isoprene butyl rubber by cationic polymerization of the corresponding monomers at temperatures of from about -90.degree. C. to -20.degree. C. utilizing as the catalyst AlCl.sub.3 or aluminum ethyl dichloride in methyl chloride, methylene chloride, ethyl chloride or vinyl chloride diluent utilizing the stabilizer polymers of the present invention. Heretofore, it has simply not been possible to prepare non-agglomerating butyl rubber at temperatures warmer than about -90.degree. C. Furthermore, maintenance of a stable polymerization slurry at such temperatures enables the use of a wide variety of catalysts other than AlCl.sub.3 to become practicable.